Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.

Doubtless can be an adjective meaning “free of doubt,” but it is relatively rare: He is doubtless and fearless when he begins his performance. Doubtless is also an adverb, as are all the other locutions in this entry; all mean “probably,” “certainly,” or “absolutely.” Doubtless and doubtlessly (which is rare and often considered clumsy since the flat adverb doubtless is available too) may not always sound very certain; they usually express the sense of “probably, very likely.” No doubt is perhaps a bit stronger, but not quite certain in some contexts; in others it shares the force of undoubtedly. Undoubtedly and without doubt are stronger; they express certainty. Indubitably and unquestionably are the most forceful of all. They express dead certainty, leaving not the shadow of a doubt. Try each of them in place of doubtless in this sentence for an idea of the gradations: It will doubtless rain this afternoon.